The present invention relates to a valve-timing control method and apparatus for controlling the timing of a valve of an engine.
An internal combustion engine is an apparatus for generating power by taking in air and fuel, and burning them in a combustion chamber. The engine is equipped with an intake valve for taking the air and fuel into the combustion chamber, and an exhaust valve for exhausting the burned gas from the combustion chamber. The intake and exhaust valves are usually driven by one or more camshafts.
The valve timing controls when the valves open and close with respect to the ignition spark. Preferable valve timing (referred as target timing hereinafter) may depend on engine revolution speed, engine load, etc. Therefore, variable valve-timing (VVT) apparatuses have recently been developed wherein the rotation of the camshaft is not fixed with respect to the rotation of a crankshaft, and it has relative rotational movement within a predetermined range.
A continuously variable valve-timing (CVVT) apparatus, a kind of VVT apparatus, can control the valve timing continuously within a particular range. A conventional CVVT apparatus includes a rotary piston arrangement whereby the position of the camshaft may be advanced or delayed through hydraulic control. Typically, the hydraulic pressure is controlled by means including a duty-controlled solenoid valve. The solenoid valve is in turn controlled by an electronic control unit (ECU) such that the valve timing can be continuously controlled by duty-control of the solenoid valve.
In order to control the valve timing, the ECU changes the duty-ratio to one corresponding to target timing. To this end, according to the prior art, the ECU repeatedly changes the duty-ratio of the solenoid valve by a predetermined change value until the target timing is realized.
However, according to this simple method for controlling valve timing, significant time is consumed in changing the valve timing to the target timing. Also, if the predetermined change value is preset at a high value, an abrupt change in valve timing may cause misfire.
Examples of characteristic curves for valve timing in relation to the duty-ratio of the solenoid valve, with respect to a plurality of oil temperatures, are shown in FIG. 2. In this figure the horizontal axis denotes the duty-ratio of the solenoid valve, and the vertical axis denotes valve timing. The amount of fluid provided to the CVVT apparatus under control of the solenoid valve depends on fluid temperature and battery voltage. As shown in FIG. 2, resultant valve timing is not proportional to the varying duty-ratio of the solenoid valve. Instead its dependency forms a curve with a flat region near a certain valve timing.
Therefore, for example referring to the curve of the fluid temperature being xe2x88x9220 deg. C., the resultant valve timing does not substantially change while the duty-ratio is being repeatedly increased from a point A to a point B and vice versa. This means that a slow response of valve timing control occurs from the point A to the point B, and vice versa.
However, if the predetermined change value is increased, the valve timing control loses precision. And if the duty-ratio is abruptly changed to below point B in the case that the target timing is near point B, sudden retarding occurs which may cause misfire. Accordingly, there is a need in the art for a continuously variable valve-timing control method and apparatus that enables more rapid and more precise control of the timing of a valve of an engine.
An exemplary valve-timing control method according to an embodiment of the present invention includes measuring a current valve timing, determining a target timing for the valve timing, initially-adjusting a current duty-ratio of an actuator adapted to vary the valve timing by a learned initial value, and gradually-adjusting the current duty-ratio until the current valve timing corresponds to the target timing.
The gradually-adjusting of the current duty-ratio may include detecting a current valve timing, determining if the current valve timing corresponds to the target timing, and adjusting the current duty-ratio of the actuator by a predetermined value when the current valve timing does not correspond to the target timing. The current valve timing is considered as corresponding to the target timing when the current valve timing is within a predetermined range of the target timing.
When the adjusting of the current duty-ratio is completed, the detecting a current valve timing is executed such that the current duty-ratio is gradually adjusted. The gradually-adjusting the current duty-ratio stops when the valve timing corresponds to the target timing.
The learned initial value is preferably retrieved from a map having independent variables including fluid temperature, with the fluid being used for control of the valve timing. The initial value is preferably chosen from a plurality of values including an initial advance value and an initial retard value. The initial advance value is preferably an initial value of adjustment when the target timing is advanced from the current valve timing, and the initial retard value is preferably an initial value of adjustment when the target timing is retarded from the current valve timing.
The method of the present invention may further include determining if a condition for learning the initial value is satisfied, learning the initial advance value when the condition for learning the initial value is satisfied, learning the initial retard value when the condition for learning the initial value is satisfied, and storing the initial advance value and initial retard value, such that the initial value is preferably learned. The condition for learning the initial value may be satisfied if the fluid temperature of the control fluid lies within a predetermined temperature range, the battery voltage lies within a predetermined voltage range, and the current valve timing lies within a predetermined timing range.
Learning the initial advance value may comprise recursively adjusting the current duty-ratio of the actuator by a predetermined advance increment until the valve timing is advanced by more than a predetermined timing amount. In that case, the initial advance value is preferably learned as a difference between the initial current duty-ratio before recursive adjustment and the current duty-ratio recursively adjusted to just before the valve timing is advanced by more than the predetermined timing amount.
Learning the initial retard value may comprise recursively adjusting the current duty-ratio of the actuator by a predetermined retard increment until the valve timing is retarded by more than a predetermined timing amount. In that case, the initial retard value is preferably learned as a difference between the initial current duty-ratio before recursive adjustment and the current duty-ratio recursively adjusted to just before the valve timing is retarded by more than the predetermined timing amount. Preferably the initial values are stored based on parameters including the fluid temperature and the battery voltage.
In a further aspect of the invention a valve-timing control apparatus is provided. In one embodiment, a valve-timing detector is adapted to detect valve timing. An advance chamber and a retard chamber advance and retard the valve timing by inflow of hydraulic fluid. A hydraulic pump generates hydraulic pressure supplied to the advance and retard chambers. A proportional valve controls a proportion of the generated hydraulic pressure supplied to the advance chamber and retard chamber. An actuator controls operation of the proportional valve based on signals from an electronic control unit which is programmed to execute predetermined instructions for duty-control of the actuator.
The valve-timing control apparatus preferably further includes a temperature detector for detecting fluid temperature and a voltage detector for detecting battery voltage. The electronic control unit can be realized by one or more microprocessors in which a program for executing the predetermined instructions is implemented.